'\"macro stdmacro
.\"
.\" Copyright (c) 2013 Red Hat.
.\" Copyright (c) 2000-2004 Silicon Graphics, Inc.  All Rights Reserved.
.\"
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.\" under the terms of the GNU General Public License as published by the
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.TH PMDAMAIN 3 "PCP" "Performance Co-Pilot"
.SH NAME
\f3pmdaMain\f1,
\f3pmdaGetContext\f1,
\f3pmdaSetResultCallBack\f1,
\f3pmdaSetCheckCallBack\f1,
\f3pmdaSetDoneCallBack\f1,
\f3pmdaSetEndContextCallBack\f1 \- generic PDU processing for a PMDA
.SH "C SYNOPSIS"
.ft 3
.ad l
.hy 0
#include <pcp/pmapi.h>
.br
#include <pcp/pmda.h>
.sp
void pmdaMain(pmdaInterface *\fIdispatch\fP);
.br
void pmdaSetCheckCallBack(pmdaInterface *\fIdispatch\fP,
'in +\w'void pmdaSetCheckCallBack('u
pmdaCheckCallBack\ \fIcallback\fP);
.in
.br
void pmdaSetDoneCallBack(pmdaInterface *\fIdispatch\fP,
'in +\w'void pmdaSetDoneCallBack('u
pmdaDoneCallBack\ \fIcallback\fP);
.in
.br
void pmdaSetResultCallBack(pmdaInterface *\fIdispatch\fP,
'in +\w'void pmdaSetResultCallBack('u
pmdaResultCallBack\ \fIcallback\fP);
.in
.br
void pmdaSetEndContextCallBack(pmdaInterface *\fIdispatch\fP,
'in +\w'void pmdaSetEndContextCallBack('u
pmdaEndContextCallBack\ \fIcallback\fP);
.in
.br
int pmdaGetContext(void);
.sp
cc ... \-lpcp_pmda \-lpcp
.hy
.ad
.ft 1
.SH DESCRIPTION
For Performance Metric Domain Agents
.RB ( PMDA (3))
using the binary PDU protocols to communicate with
.BR pmcd (1),
the routine
.B pmdaMain
provides a generic implementation of the PDU-driven main loop.
.PP
.I dispatch
describes how to process each incoming PDU. It
is a vector of function pointers, one per request PDU type,
as used in the DSO interface for a PMDA, namely:
.PP
.nf
.ft CR
/*
 * Interface Definitions for PMDA Methods
 */
typedef struct {
    int domain;         /* set/return performance metrics domain id here */
    struct {
        unsigned int    pmda_interface: 8; /* PMDA DSO interface version */
        unsigned int    pmapi_version : 8; /* PMAPI version */
        unsigned int    flags : 16;        /* optional feature flags */
    } comm;             /* set/return communication and version info */
    int status;         /* return initialization status here */

    union {
        struct {                              /* PMDA_INTERFACE_2 or _3 */
            pmdaExt *ext;
            int (*profile)(pmProfile *, pmdaExt *);
            int (*fetch)(int, pmID *, pmResult **, pmdaExt *);
            int (*desc)(pmID, pmDesc *, pmdaExt *);
            int (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
            int (*text)(int, int, char **, pmdaExt *);
            int (*store)(pmResult *, pmdaExt *);
        } two, three;

        struct {                              /* PMDA_INTERFACE_4 or _5 */
            pmdaExt *ext;
            int     (*profile)(pmProfile *, pmdaExt *);
            int     (*fetch)(int, pmID *, pmResult **, pmdaExt *);
            int     (*desc)(pmID, pmDesc *, pmdaExt *);
            int     (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
            int     (*text)(int, int, char **, pmdaExt *);
            int     (*store)(pmResult *, pmdaExt *);
            int     (*pmid)(char *, pmID *, pmdaExt *);
            int     (*name)(pmID, char ***, pmdaExt *);
            int     (*children)(char *, int, char ***, int **, pmdaExt *);
        } four, five;

        struct {                              /* PMDA_INTERFACE_6 */
            pmdaExt *ext;
            int     (*profile)(pmProfile *, pmdaExt *);
            int     (*fetch)(int, pmID *, pmResult **, pmdaExt *);
            int     (*desc)(pmID, pmDesc *, pmdaExt *);
            int     (*instance)(pmInDom, int, char *, pmInResult **, pmdaExt *);
            int     (*text)(int, int, char **, pmdaExt *);
            int     (*store)(pmResult *, pmdaExt *);
            int     (*pmid)(char *, pmID *, pmdaExt *);
            int     (*name)(pmID, char ***, pmdaExt *);
            int     (*children)(char *, int, char ***, int **, pmdaExt *);
            int     (*attribute)(int, int, const char *, int, pmdaExt *);
        } six;
    } version;

} pmdaInterface;
.fi
.PP
This structure has been extended to incorporate the multiple interface versions
that have evolved over time.
For
.BR pmdaMain,
.I dispatch->domain
and
.I dispatch->status
are ignored.  The
.I comm.pmda_interface
field is used to determine the interface used by the PMDA.  Setting this field
to
.B PMDA_INTERFACE_2
or
.B PMDA_INTERFACE_3
will force
.B pmdaMain
to use the callbacks in the
.I version.two
or
.I version.three
structure.
A setting of
.B PMDA_INTERFACE_4
or
.B PMDA_INTERFACE_5
will force
.B pmdaMain
to use the callbacks in the
.I version.four
or
.I version.five
structure, and similarly a
.B PMDA_INTERFACE_6
setting forces
.B pmdaMain
to use the callbacks in the
.I version.six
structure.
Any other value will result in an error and termination of
.BR pmdaMain .
.PP
Note that the use of
.B dispatch
as the interface between the
.BR pmcd (1)
and the methods of the PMDA allows each PMDA to be implemented as
though it were a DSO, with
.B pmdaMain
providing a convenient wrapper that may be used to convert from the
DSO interface to the binary PDU (daemon PMDA) interface.
.PP
.B pmdaMain
executes as a continuous loop, returning only when an end of file
is encountered on the PDU input file descriptor.
.SH CALLBACKS
In addition to the individual PDU processing callbacks \-
.BR pmdaProfile (3),
.BR pmdaFetch (3),
.BR pmdaDesc (3),
.BR pmdaInstance (3),
.BR pmdaText (3),
.BR pmdaStore (3),
.BR pmdaPMID (3),
.BR pmdaName (3),
.BR pmdaChildren (3),
and
.BR pmdaAttribute (3)
there are other callbacks that can affect or inform all PDU
processing within a PMDA, namely
.IR check ,
.I done
and
.IR end .
These callbacks should be set with
.BR pmdaSetCheckCallBack ,
.B pmdaSetDoneCallBack
and
.BR pmdaSetEndContextCallBack .
.PP
If not null,
.I check
is called after each PDU is received (but before it was processed), and
.I done
is called after each PDU is sent.
If
.I check
returns a value less than zero (typically PM_ERR_AGAIN),
the PDU processing is skipped and in most cases the
function value is returned as an error PDU to
.BR pmcd (1)
\- this may be used for
PMDAs that require some sort of deferred connection or reconnect
protocols for the underlying sources of performance metrics, e.g. a DBMS.
The error indication from
.I check
is not passed back to
.BR pmcd (1)
in the cases where no acknowledgment is expected, e.g. for a PDU_PROFILE.
.PP
The
.I end
callback allows a PMDA to keep track of state for individual clients that
are requesting it to perform actions (PDU processing).
Using
.B pmdaGetContext
a PMDA can determine, at any point, an integer identifier that uniquely
identifies the client tools at the remote end of PMCD (for local context
modes, this identifier is always zero).
This becomes very important for handling event metrics, where each
event must be propagated once only to each interested client.
It also underlies the mechanism whereby connection information is passed
to the PMDA, such as the the credentials (user and group identifiers) for
the client tool.
.PP
One final callback mechanism is provided for handling the
.B pmResult
built for a PDU_RESULT in response to a PDU_FETCH request.
By default,
.B pmdaMain
will free the
.B pmResult
once the result has been sent to the
.BR pmcd (1).
For some PMDAs this is inappropriate, e.g. the
.B pmResult
is statically allocated, or contains a hybrid of pinned PDU buffer
information and dynamically allocated information.
.B pmdaSetResultCallBack
may be used to define an alternative
.B callback
from
.BR pmdaMain .
.SH DIAGNOSTICS
These messages may be appended to the PMDA's log file:
.TP 25
.BI "PMDA interface version " interface " not supported"
The
.I interface
version is not supported by
.BR pmdaMain .
.TP
.B Unrecognized pdu type
The PMDA received a PDU from
.B pmcd
that it does not recognize. This may indicate that the
.B pmcd
process is using a more advanced interface than
.BR pmdaMain .
.PP
If the
.BR PMAPI (3)
debugging control options
have the ``libpmda'' option set then each PDU that is received is reported
in the PMDA's log file.
.SH SEE ALSO
.BR pmcd (1),
.BR PMAPI (3),
.BR PMDA (3),
.BR pmdaProfile (3),
.BR pmdaFetch (3),
.BR pmdaDesc (3),
.BR pmdaInstance (3),
.BR pmdaText (3),
.BR pmdaStore (3),
.BR pmdaPMID (3),
.BR pmdaName (3),
.BR pmdaChildren (3),
and
.BR pmdaAttribute (3).

.\" control lines for scripts/man-spell
.\" +ok+ pmda_interface pmapi_version libpmda DBMS desc comm pdu ext
